A phase II study of troglitazone, an activator of the PPARgamma receptor, in patients with chemotherapy-resistant metastatic colorectal cancer

Peroxisome proliferator-activated receptor-gamma ligands for the treatment of breast cancer

Pioglitazone and rosiglitazone are thiazolidinediones used for the treatment of Type 2 diabetes mellitus. They modulate glucose and fat metabolism, mainly by binding to the nuclear hormone receptor peroxisome proliferator-activated receptor (PPAR)-gamma. PPAR-gamma signalling is involved in a number of other disease conditions including cancer. In breast cancer cells, PPAR-gamma ligands inhibit proliferation and induce apoptosis both in vitro and in vivo. PPAR-gamma ligands also inhibit tumour angiogenesis and invasion. The only published clinical trial using a PPAR-gamma ligand in patients with metastatic breast cancer failed to show any clinical benefits. The mechanism of action of the thiazolidinediones in breast cancer cells is not fully understood but involves interactions with other nuclear hormone receptors, transcriptional co-activators and repressors as well as PPAR-gamma-independent effects. A better understanding of these mechanisms will be needed before PPAR-gamma ligands may be useful in the treatment of breast cancer patients.

Bioassay for the identification of natural product-based activators of peroxisome proliferator-activated receptor-gamma (PPARgamma): the marine sponge metabolite psammaplin A activates PPARgamma and induces apoptosis in human breast tumor cells

Peroxisome proliferator-activated receptors (PPARs), members of the nuclear hormone receptor (NHR) family, are ligand-activated transcription factors. Ligands (agonists) of PPARgamma have been shown to inhibit growth, promote terminal differentiation, and induce apoptosis in human breast tumor cells. A cell-based reporter assay was developed to examine extracts of terrestrial and marine organisms for the ability to activate PPARgamma. Bioassay-guided fractionation and isolation of an active extract from Pseudoceratina rhax yielded the known histone deacetylase (HDAC) inhibitor psammaplin A (1). Compound 1 activates PPARgamma in a MCF-7 cell-based reporter assay and induces apoptosis in human breast tumor cells in vitro. Molecular modeling studies suggest that 1 may interact with binding sites within the PPARgamma ligand-binding pocket. Therefore, in addition to its known effects on HDAC-mediated processes, activation of PPARgamma-regulated gene expression may play a role in the ability of 1 to induce apoptosis.

PPARbeta/delta selectively induces differentiation and inhibits cell proliferation

Peroxisome proliferator-activated receptor (PPAR) beta-null mice exhibit exacerbated epithelial cell proliferation and enhanced sensitivity to skin carcinogenesis, suggesting that ligand activation of PPARbeta will inhibit keratinocyte proliferation. By using of a highly specific ligand (GW0742) and the PPARbeta-null mouse model, activation of PPARbeta was found to selectively induce keratinocyte terminal differentiation and inhibit keratinocyte proliferation. Additionally, GW0742 was found to be anti-inflammatory due to inhibition of myeloperoxidase activity, independent of PPARbeta. These data suggest that ligand activation of PPARbeta could be a novel approach to selectively induce differentiation and inhibit cell proliferation, thus representing a new molecular target for the treatment of skin disorders resulting from altered cell proliferation such as psoriasis and cancer.

Dramatic synergistic anticancer effect of clinically achievable doses of lovastatin and troglitazone

Lovastatin (an HMG-CoA reductase inhibitor) and troglitazone (a PPAR-gamma agonist) have been intensively studied prospectively for their application in cancer treatment. However, clinical trials of lovastatin or troglitazone in cancer treatment resulted in only limited responses. To improve their efficacy, lovastatin and troglitazone have, respectively, been tried to combine with other anticancer agents with varied outcomes. In our study, we found a dramatic synergism between lovastatin and troglitazone in anticancer at clinically achievable concentrations. This synergism was found in far majority of cell lines tested including DBTRG 05 MG (glioblastoma) and CL1-0 (lung). This amazing synergism was accompanied by synergistic modulation of E2F-1 and p27(Kip1), which were reported to mediate the anticancer activities of lovastatin and troglitazone, respectively, and other cell cycle regulating proteins such as CDK2, cyclin A and RB phosphorylation status. With this dramatic combination effect of lovastatin and troglitazone, a promising regimen of cancer therapy may be materialized in the future.

Gene-expression profiling in differentiated thyroid cancer – a viable strategy for the practice of genomic medicine?

Thyroid neoplasias have been largely ignored as an active field of investigation due to the overall favorable prognosis of differentiated nonmedullary thyroid cancers. However, differentiated thyroid cancers have the highest estimated annual percentage increase in incidence amongst all cancer sites. Furthermore, no significant progress has been made to improve survival, especially for advanced disease. Compounding the problem, there remains a lack of highly accurate preoperative markers or molecular-based predictive models to differentiate benign from malignant follicular neoplasias, thus we continue to rely upon surgery for diagnostic purposes in this subset of patients. Therefore, new approaches are necessary to identify potential novel diagnostic, prognostic and therapeutic algorithms, which would not only allow accurate early diagnosis but also personalized patient management, with clinical management and surveillance tailored according to the genetic signature of the patient. The advent of modern genomic technologies, such as global gene-expression profiling, may begin to provide the data required for the evidence-based practice of genomic medicine as it relates to thyroid neoplasia. However, it is already clear that genomic technology alone is insufficient to fully achieve this vision.

The PPAR(gamma) K422Q mutation does not contribute to troglitazone inefficiency in colon cancer treatment

Peroxisome proliferator-activated receptor gamma (PPAR(gamma)) ligands inhibit cell growth of colorectal cancer cells in most experimental models, but no significant effect could be observed in patients with colorectal cancer. We therefore, screened human colorectal tumors to determine the prevalence of the PPAR(gamma) K422Q loss-of-function mutation, recently identified in 50% of colonic cancer cell lines. A sensitive allele-specific real-time amplification assay was developed and 170 colorectal primary tumors and 12 liver metastasis were analyzed. We did not find the K422Q mutation in any of these samples. We can therefore exclude this alteration as a mechanism of resistance to PPAR(gamma) ligands in patients with colon cancer.

Peroxisome Proliferator‐activated Receptors and their Relevance to Dermatology

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily and are expressed in a variety of tissues including skin and cells of the immune system. They act as ligand-dependent transcription factors which heterodimerize with retinoid X receptors to allow binding to and activation of PPAR responsive genes. Through this mechanism, PPAR ligands can control a wide range of physiological processes. Based on their effects in vitro and in vivo PPAR agonists and antagonists have the potential to become important therapeutic agents for the treatment of various skin diseases. PPARs can also be activated directly by phosphorylation to have ligand-independent effects. This review will discuss the physiology of PPARs relating this to skin pathology and their role as a target for novel therapies.

Peroxisome proliferator-activated receptor gamma inhibition prevents adhesion to the extracellular matrix and induces anoikis in hepatocellular carcinoma cells

Activation of the nuclear transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits growth and survival of hepatocellular carcinoma (HCC) cell lines. To further investigate the function of PPARgamma in HCC, PPARgamma expression patterns in primary tumors were examined, and the responses of two HCC cell lines to PPARgamma activation and inhibition were compared. PPARgamma expression was increased in HCC and benign-appearing peritumoral hepatocytes compared with remote benign hepatocytes. Both compound PPARgamma inhibitors and PPARgamma small interfering RNAs prevented HCC cell lines from adhering to the extracellular matrix. Loss of adhesion was followed by caspase-dependent apoptosis (anoikis). PPARgamma inhibitors had no effect on initial beta1 integrin-mediated adhesion, or on total focal adhesion kinase levels but did reduce focal adhesion kinase phosphorylation. The PPARgamma inhibitor T0070907 was significantly more efficient at causing cancer cell death than the activators troglitazone and rosiglitazone. T0070907 caused cell death by reducing adhesion and inducing anoikis, whereas the activators had no direct effect on adhesion and caused cell death at much higher concentrations. In conclusion, PPARgamma overexpression is present in HCC. Inhibition of PPARgamma function causes HCC cell death by preventing adhesion and inducing anoikis-mediated apoptosis. PPARgamma inhibitors represent a potential novel treatment approach to HCC.

Hic-5 regulates an epithelial program mediated by PPARgamma

PPARgamma is a dominant regulator of fat cell differentiation. However, this nuclear receptor also plays an important role in the differentiation of intestinal and other epithelial cell types. The mechanism by which PPARgamma can influence the differentiation of such diverse cell lineages is unknown. We show here that PPARgamma interacts with Hic-5, a coactivator protein expressed in gut epithelial cells. Hic-5 and PPARgamma colocalize to the villus epithelium of the small intestine, and their expression during embryonic gut development correlates with the transition from endoderm to a specialized epithelium; expression of both these factors is reduced in tumors. Forced expression of Hic-5 in colon cancer cells enhances the PPARgamma-mediated induction of several gut epithelial differentiation/maturation markers such as L-FABP, kruppel-like factor 4 (KLF4), and keratin 20. siRNA directed against Hic-5 specifically reduces PPARgamma-mediated induction of gut epithelial genes in colon cells and in an ex vivo model of embryonic gut differentiation. Finally, forced expression of Hic-5 during 3T3-L1 preadipocyte differentiation inhibits adipogenesis while inducing inappropriate expression of several mRNAs characteristic of gut epithelium in these mesenchymal cells. These results indicate that Hic5 is an important component in determining an epithelial differentiation program induced by PPARgamma.

Review article: the potential of combinational regimen with non-steroidal anti-inflammatory drugs in the chemoprevention of colorectal cancer

Non-steroidal anti-inflammatory drugs are chemopreventive agents in colorectal cancer. Non-steroidal anti-inflammatory drugs do not, however, offer complete protection against adenoma and carcinoma development. There is increasing interest in combining non-steroidal anti-inflammatory drugs with agents that target specific cell signalling pathways in malignant and premalignant cells. This review aims to describe the current knowledge regarding the efficacy of peroxisome proliferator-activated receptor-gamma ligands, cholesterol synthesis inhibitors (statins), epidermal growth factor signalling inhibitors and tumour necrosis factor-related apoptosis-inducing ligand against colorectal neoplasms and the rationale for combining these drugs with non-steroidal anti-inflammatory drugs to improve efficacy in the chemoprevention of colorectal cancer, a PUBMED computer search of the English language literature was conducted to identify relevant papers published before July 2004. Peroxisome proliferator-activated receptor-gamma ligands and statins, both in clinical use, reduce the growth rate of human colon cancer cells in vitro and in rodents models. In vitro, preclinical in vivo and clinical studies have shown efficacy of epidermal growth factor signalling inhibition in colorectal cancer. In vitro, tumour necrosis factor-related apoptosis-inducing ligand induces apoptosis in human colon cancer cells, but not in normal cells. These drugs have all been shown to interact with non-steroidal anti-inflammatory drugs in colorectal cancer cells and/or in rodent models. Combinational regimen are a promising strategy for the chemoprevention of colorectal cancer and should be further explored.

Peroxisome proliferator-activated receptor-γ (PPARγ) inhibits tumorigenesis by reversing the undifferentiated phenotype of metastatic non-small-cell lung cancer cells (NSCLC)

Pharmacological activators of peroxisome proliferator-activated receptor-gamma (PPAR(gamma)) have been shown to inhibit growth of lung tumors largely through growth inhibition and induction of apopotosis. However, since many of these agents engage other effectors, the role of (PPAR(gamma) in lung tumorigenesis remains poorly defined. To specifically examine PPAR(gamma)-mediated events, non-small-cell lung cancer (NSCLC) cells overexpressing PPAR(gamma) were established. Overexpression of PPAR(gamma) in H2122 adenocarcinoma cells (H2122-PPAR(gamma)) blocked anchorage-independent growth compared to cells transfected with empty vector (H2122-LNCX), but had no significant effect on cell proliferation or apoptosis under standard tissue culture conditions. Orthotopic implantation of H2122-PPAR(gamma) cells into the lungs of nude rats inhibited tumor growth and metastasis in vivo and prolonged survival compared to implantation of H2122-LNCX cells. Consistent with these findings, H2122-PPAR(gamma) cells had an impaired invasiveness as assessed in Transwell assays. In a three-dimensional culture system, H2122-PPAR(gamma) cells formed polarized spheroid structures similar to those observed with normal lung epithelial cells. H2122-LNCX cells formed nonpolarized aggregate structures and did not show any of these epithelial properties. These data indicate that inhibitory effects of PPAR(gamma) on lung tumorigenesis involve selective inhibition of invasive metastasis, and activation of pathways that promote a more differentiated epithelial phenotype.

Antineoplastic effects of peroxisome proliferator-activated receptor gamma agonists

Peroxisome proliferator-activated receptors (PPAR) are members of a superfamily of nuclear hormone receptors. Activation of PPAR isoforms elicits both antineoplastic and anti-inflammatory effects in several types of mammalian cells. PPARs are ligand-activated transcription factors and have a subfamily of three different isoforms: PPAR alpha, PPAR gamma, and PPAR beta/delta. All isoforms heterodimerise with the 9-cis-retinoic acid receptor RXR, and play an important part in the regulation of several metabolic pathways, including lipid biosynthesis and glucose metabolism. Endogenous ligands of PPAR gamma include long-chain polyunsaturated fatty acids, eicosanoid derivates, and oxidised lipids. Newly developed synthetic ligands include thiazolidinediones-a group of potent PPAR gamma agonists and antidiabetic agents. Here, we review PPAR gamma-induced antineoplastic signalling pathways, and summarise the antineoplastic effects of PPAR gamma agonists in different cancer cell lines, animal models, and clinical trials.

Redifferentiation therapy for thyroid cancer

Thyroid tumorigenesis and carcinogenesis accompany progressive loss of thyroid-specific differentiated functions. Some thyroid cancers are or become dedifferentiated, and they become refractory to efficacy-proven conventional therapies such as radioiodine ablation therapy and thyrotropin (TSH)-suppressive therapy. Redifferentiation therapy by either redifferentiating agents or gene transfer of differentiation-related genes may retard tumor growth and make tumors respond to conventional therapies.

Peroxisome proliferator-activated receptor-delta attenuates colon carcinogenesis

Peroxisome proliferator-activated receptor-delta (PPAR-delta; also known as PPAR-beta) is expressed at high levels in colon tumors, but its contribution to colon cancer is unclear. We examined the role of PPAR-delta in colon carcinogenesis using PPAR-delta-deficient (Ppard(-/-)) mice. In both the Min mutant and chemically induced mouse models, colon polyp formation was significantly greater in mice nullizygous for PPAR-delta. In contrast to previous reports suggesting that activation of PPAR-delta potentiates colon polyp formation, here we show that PPAR-delta attenuates colon carcinogenesis.

Therapeutic potential of thiazolidinediones as anticancer agents

Thiazolidinediones (TZDs) are synthetic ligands that activate the nuclear receptor peroxisome proliferator-activated receptor-gamma (PPAR-gamma). These compounds are widely used in the treatment of Type 2 diabetes. TZDs have antitumour activity in a wide variety of experimental cancer models, in vitro and in vivo, by affecting the cell cycle, induction of cell differentiation and apoptosis as well as by inhibiting tumour angiogenesis. These effects are mediated through both PPAR-gamma-dependent and -independent pathways depending on concentration and tumour cell type. Angiogenesis inhibition mechanisms of TZDs include directly inhibiting endothelial cell proliferation and migration as well as decreasing tumour cell vascular endothelial growth factor production. Further studies suggest that TZDs may be effective in prevention of certain cancers and in the treatment of cancer as adjuvant therapy.

A phase II trial with rosiglitazone in liposarcoma patients

Agents of the thiazolidinedione drug family can terminally differentiate human liposarcoma cells in vitro by activating genes responsible for lipocyte differentiation. One study has shown clinical activity of troglitazone treatment in liposarcoma patients. We sought to find further evidence for this result. In all, 12 patients with a liposarcoma received rosiglitazone 4 mg b.d. They were followed clinically and with repeated biopsies for histological and biological studies. At the molecular level the mRNA translation of three genes that are induced by this treatment (peroxisome proliferator-activated receptor γ (PPARγ), adipsin and fatty acid binding protein) was determined. Nine patients were eligible for evaluation. One patient had to stop treatment due to hepatotoxicity. The mean time to progression was 6 months (2 – 16 months), with one patient still on treatment. We did not see any significant change in histologic appearance of the liposarcomas by the treatment. The level of gene expression changed significantly in two patients, but this did not result in a clinical response. Based on this study, rosiglitazone is not effective as an antitumoral drug in the treatment of liposarcomas. Increased PPARγ activity does not correlate with the clinical evolution.

Perspective: embedded molecular switches, anticancer selection, and effects on ontogenetic rates: a hypothesis of developmental constraint on morphogenesis and evolution

The switch between the cell cycle and the progress of differentiation in developmental pathways is prevalent throughout the eukaryotes in all major cell lineages. Disruptions to the molecular signals regulating the switch between proliferative and differentiating states are severe, often resulting in cancer formation (uncontrolled proliferation) or major developmental disorders. Uncontrolled proliferation and developmental disorders are potentially lethal defects in the developing animal. Therefore, natural selection would likely favor a tightly controlled regulatory mechanism to help prevent these fundamental defects. Although selection is usually thought of as a consequence of environmental or ecological influences, in this case the selective force to maintain this molecular switch is internal, manifested as a potentially lethal developmental defect. The morphogenetic consequences of this prevalent, deeply embedded, and tightly controlled mechanistic switch are currently unexplored, however experimental and correlative evidence from several sources suggest that there are important consequences on the control of growth rates and developmental rates in organs and in the whole animal. These observations lead one to consider the possibility of a developmental constraint on ontogenetic rates and morphological evolution maintained by natural selection against cancer and other embryonic lethal defects.

Advances in understanding the regulation of apoptosis and mitosis by peroxisome-proliferator activated receptors in pre-clinical models: relevance for human health and disease

Peroxisome proliferator activated receptors (PPARs) are a family of related receptors implicated in a diverse array of biological processes. There are 3 main isotypes of PPARs known as PPARalpha, PPARbeta and PPARgamma and each is organized into domains associated with a function such as ligand binding, activation and DNA binding. PPARs are activated by ligands, which can be both endogenous such as fatty acids or their derivatives, or synthetic, such as peroxisome proliferators, hypolipidaemic drugs, anti-inflammatory or insulin-sensitizing drugs. Once activated, PPARs bind to DNA and regulate gene transcription. The different isotypes differ in their expression patterns, lending clues on their function. PPARalpha is expressed mainly in liver whereas PPARgamma is expressed in fat and in some macrophages. Activation of PPARalpha in rodent liver is associated with peroxisome proliferation and with suppression of apoptosis and induction of cell proliferation. The mechanism by which activation of PPARalpha regulates apoptosis and proliferation is unclear but is likely to involve target gene transcription. Similarly, PPARgamma is involved in the induction of cell growth arrest occurring during the differentiation process of fibroblasts to adipocytes. However, it has been implicated in the regulation of cell cycle and cell proliferation in colon cancer models. Less in known concerning PPARbeta but it was identified as a downstream target gene for APC/beta-catenin/T cell factor-4 tumor suppressor pathway, which is involved in the regulation of growth promoting genes such as c-myc and cyclin D1. Marked species and tissue differences in the expression of PPARs complicate the extrapolation of pre-clinical data to humans. For example, PPARalpha ligands such as the hypolipidaemic fibrates have been used extensively in the clinic over the past 20 years to treat cardiovascular disease and side effects of clinical fibrate use are rare, despite the observation that these compounds are rodent carcinogens. Similarly, adverse clinical responses have been seen with PPARgamma ligands that were not predicted by pre-clinical models. Here, we consider the response to PPAR ligands seen in pre-clinical models of efficacy and safety in the context of human health and disease.